A loss of topsoil can result in a significant loss in
productivity, largely due to losses of organic matter and nutrients
as well as deterioration of physical soil properties.
“It was found that yields generally decreased
as the amount of topsoil removed increased. Data
indicated yields to be severely depressed on all
topsoil removal treatments where no fertilizer was
applied. …On the coarse textured soils, even twice
the recommended rate of fertilizer was not able to
bring the yields back to that of the control.”
(Kapoor and Shaykewich, 1990;
Kenyon and Shaykewich, 1987).
A. Wind erosion
Wind erosion is the detachment, movement and depletion of soil
from the land surface by wind. It can occur naturally, without human
intervention, or can be accelerated through human activities such as
excessive tillage.
Soils most susceptible to wind erosion by texture:
sands > clays > loams
Soil most susceptible to wind erosion by structure:
single-grained (structureless) > crumbly or cloddy
- Maximum tolerable loss: 5 tons/acre/year (10 tonnes/hectare/year)
= 0.03 inches (0.75 millimetres) thickness of topsoil on
a well-developed soil
- Pulse crops and potatoes usually do not leave enough
residue on the surface to prevent erosion once these
fields are cultivated – these crops are usually grown on
the most erodible soil types.
- A 30 mph (48 km/h) wind has more than 3 times more
erosive power than a 20 mph (32 km/h) wind.
- Wind erosion increases as soil dries (eg. air-dry
soil erodes 1.3 times faster than soil at permanent
wilting point).
- The most susceptible period for soil erosion by wind
is early spring and after fall tillage.
- Soil particles move by wind in one of three ways:
surface creep (rolling or sliding along surface);
saltation (bouncing and dislodging other
particles on impact); and suspension
(continuously carried in the air).
|
![](/web/20061121033645im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbe01s08a.jpg)
Figure 7.1 Three types of movement of soil
particles by wind erosion: surface creep, saltation and
suspension |
Consult soils report to assess risk of wind erosion
Look for items that indicate soil susceptibility to erosion:
- texture (see above)
- agriculture capability subclass E (erosion limitation)
- Eroded phases on map: 1xxx, 2xxx, 3xxx,
oxxx. Note the first position in the denominator of the
soil code refers to the degree of erosion.
xxxx = non-eroded or minimal erosion
1xxx = slightly eroded (25-75% of A horizon removed)
2xxx = moderately eroded (>75% of A and part of B horizon
removed)
3xxx = severely eroded (all of A and B horizons removed)
oxxx = overblown (subsoil deposited over topsoil)
Example:
DRN/xxxx = Durnan; no erosion
DRN/1xxx = Durnan, slightly eroded
Conduct site visit to assess risk/evidence of wind erosion
- Identify visual effects of past wind erosion events – blow
banks, light colored knolls, etc.
- Check the depth of black topsoil to determine if erosion or
deposition has occurred
- Identify any sandblasting of crops
- Estimate or measure crop residue cover
Method for measuring crop residue cover:
- Use any line, rope or tape that is equally divided
into 100 parts at 6- or 12-inch (15- or 30-centimetre)
spacings.
- Choose representative locations in the field.
- Stretch the line diagonally across the rows.
- Select a point on one edge of the line markings, and
observe that point
at each mark.
- Look straight down at that point. Do not count
residue smaller than 1/8 inch
(3 millimetres) diameter.
- Walk the entire length of the line. Count the total
number of marks with residue under them. That count will
be the per cent cover for the field.
- Repeat the procedure at least 5 times in different
areas of the field and average the findings.
|
![](/web/20061121033645im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbe01s08b.jpg)
Figure 7.2 10% crop residue cover |
![](/web/20061121033645im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbe01s08c.jpg) |
![](/web/20061121033645im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbe01s08d.jpg) |
Figure 7.3 35% crop residue cover |
Figure 7.4 65% crop residue cover |
a) Prevention:
- Maintain adequate crop residue cover (at least 35% cover
just after seeding for most soils, and at least 65% cover for
soils highly susceptible to soil erosion) - standing stubble is
1.6 times more effective at controlling wind erosion than flat
stubble.
- Establish cover crops – these crops should be solid seeded
at the appropriate time and seeding rate (Table 7.1).
- If it is not feasible to plant a cover crop on the entire
field, plant on headlands (field perimeter), or on/beside the
most susceptible areas.
Table 7.1 Cover crop establishment criteria
Cover Crop |
Seeding Date |
Seeding Rate (lb/ac) |
Fall rye |
August 15 - September 12 |
11-23 |
Small grain |
August 15 - September 1 |
25-30 |
Millet |
July 15 - August 15 |
10-15 |
Sweet clover |
May 1 - 15 |
6-10 |
Alfalfa |
May 1 - 15 |
6-8 |
Red clover |
May 1 - 15 |
4-6 |
- annual barriers of corn or sunflowers should be planted
perpendicular to prevailing spring winds to reduce wind erosion
after erosion-susceptible crops are harvested
Table 7.2 Annual barrier establishment criteria
Crop |
Barrier width (ft) |
Barrier spacing (ft) |
Seeding date |
Corn/ Sunflowers |
5-12 (1.5-3.6 m) |
60 (18 m) |
Normal seeding date |
Shelterbelts reduce wind velocity in the area up to 30 times
the height of trees. Plant shelterbelts perpendicular to
prevailing winds. If planting shelterbelts in the middle of a
field is not feasible due to equipment access, consider planting
shelterbelts on the north and west edges of the field perimeter
to reduce the effects of prevailing winds. Contact Prairie Farm
Rehabilitation Administration for more information on
shelterbelt design and establishment.
b) Control of blowing soils:
- Emergency tillage of heavier textured soils roughens the
land surface to reduce wind velocity and trap drifting
soils; creates or brings to the surface aggregates or clods
large enough to resist wind erosion.
- Additions of:
- crop residues (1700-2000 lb/ac (1910-2247 kg/ha) of
cereal straw on highly erodible soils) – the straw may
have to be wet or anchored to the soil by packing.
Potential drawbacks include the introduction of weed
seeds and the immobilization of nitrogen due to high C:N
ratios in the straw (see Table 8.6).
- manure (solid or liquid) may be effective, but avoid
excessive nutrient applications and nutrient losses to
water sources via leaching and runoff;
- irrigation water – add enough to moisten topsoil to
prevent movement (this is a short term fix only, and may
not be feasible if water supplies are limited)
B. Water erosion
Water erosion is the detachment, movement and depletion of
soil from the land surface by precipitation leaving the
landscape as runoff. It can occur naturally, without human
intervention, or can be accelerated through human activities
such as insufficient residue cover on soils prone to runoff.
Soil erodibility is affected by surface texture, organic
matter content, size and shape of soil aggregates and the
permeability of least permeable horizon.
Susceptibility to soil erosion by texture:
clays or loams > sands
Susceptibility to soil erosion by structure:
single-grained (structureless) > crumbly or cloddy
Rainfall quantity, intensity and duration influence the
extent of water erosion. Intense rainstorms of more than 1 inch
per hour (2.5 centimetres per hour) exceed most soils’ capacity
to absorb water, creating runoff conditions which lead to water
erosion on unprotected fields.
The degree of soil erosion is affected by slope length and
steepness - doubling the length of a slope increases soil losses
by 1.5 times; doubling the incline of a slope increases soil
losses by 2.5 times
% slope = rise X 100%
run
- Maximum tolerable loss: 5 tons/acre/year (10 tonnes/hectare/year)
= 0.03 inches (0.75 millimetres) thickness of topsoil on a
well-developed soil.
- Pulse crops and potatoes usually do not leave enough
residue on the surface to prevent erosion once these fields are
cultivated – these crops are usually grown on the most erodible
soil types.
- The most susceptible period for soil erosion by water is
during spring snowmelt and May-June, after seeding but before
canopy cover.
- Flat stubble is more effective at preventing water erosion
than standing stubble.
Consult soils report to assess risk of water erosion
Look for:
- T subclasses in the agriculture capability rating for a
given soil series or phase, indicating a slope limitation. The
exception to this would be with sandy soils, or soils with an M
(moisture) limitation. Water infiltrates faster than it can run
off on coarse textured soils, reducing the risk of water erosion
regardless of slope.
- agriculture capability subclass E (erosion limitation)
- “rapid surface runoff” in soil series description
- Slope phases on map: xbxx to xhxx. Note the second
position in the denominator of the soil code refers to the
degree of erosion.
x = 0 - 0.5% (level)
b = 0.5 - 2% (nearly level)
c = 2 - 5% (very gently sloping)
d = 5 - 9% (gently sloping)
e = 9 - 15% (moderately sloping)
f = 15 - 30% (strongly sloping)
g = 30 - 45% (very strongly sloping)
h = 45 - 70% (extremely sloping)
Example:
MXS/xxxx = Manitou; level slope
MXS/xbxx = Manitou, 0.5-2% slopes
MXS/xcxx = Manitou, >2-5% slopes
MXS/xdxx = Manitou, >5-9% slopes
MXS/xexx = Manitou, >9-15% slopes
Table 7.3 Using percent slope to make management
decisions to prevent soil erosion by water
% Slope |
Description |
Recommended Use |
% Cover Required* |
0-5% |
Level to very gentle slopes |
Annual and row crop production |
35 |
>5-9
("d" slope in soil survey reports) |
Gentle slopes |
Annual crop production |
35-50 |
>9-15 ("e") |
Moderate slopes |
Crop rotation: 2/3 forage production
1/3 annual crop production |
50-70 |
>15-30 ("f") |
Steep slopes |
Forage production |
|
>30 ("g") |
Very steep slopes |
Native production |
|
*Flat cereal residue required for effective
erosion control
Conduct site visit to assess
risk/evidence of water erosion
-
Identify visual
effects of past water erosion events – in-field channels,
gullies, etc.
-
Check the depth
of black topsoil to determine if erosion or deposition has
occurred.
-
Estimate or
measure crop residue cover
Crop management to minimize water erosion:
forages > cereals > row crops
Buffer strips of forages in sensitive areas may be appropriate.
- Establish grassed waterways with side slopes no more than
25% (1 unit rise to
4 units run); > 16 ft (4.8 m) wide, > 6 inches (15 cm) deep.
- Manage riparian areas appropriately in order to minimize
streambank erosion.
- Adopt conservation tillage practices (i.e. any tillage and
planting system that leaves at least 30% of the soil surface
covered by the previous year’s crop residue after planting).
- Consider the establishment of permanent cover – sensitive
areas may be taken out of annual crop production for forage
production, pasture production, or as a set aside for
non-agricultural uses. It may be most beneficial to establish
permanent cover on headlands or at points where soil and water
are likely to exit the property.
In hilly landscapes, tillage erosion causes the progressive
downslope movement of soil. Tillage practices which, in time,
cause more soil to be moved downslope than upslope result in the
loss of soil from the tops of hills (knolls) and the
accumulation in the bottom of hills (depressions).
Tillage erosion is described in terms of erosivity and landscape
erodibility. Large, aggressive tillage implements, operated at
excessive depths and speeds are more erosive, with more passes
resulting in more erosion. Landscapes that are very topographically
complex (with many short, steep, diverging slopes) are more
susceptible to tillage erosion.
Visual evidence of tillage erosion includes: loss of organic
rich topsoil and exposure of subsoil at the summit of ridges and
knolls; and undercutting of field boundaries, such as fence lines
and terraces, on the downslope side and burial on the upslope side.
Tillage erosion has only recently been recognized as a form of
soil erosion. Studies across North America and Europe have concluded
that tillage erosion is the major cause of the severe soil loss and
crop yield loss observed on hilltops.
The soil loss on hilltops resulting from tillage erosion reduces
crop productivity and increases field variability. Rates of soil
loss on these slope positions are often more than ten times what is
considered to be tolerable for sustainable production. Consequently,
yield losses associated with these areas are as high as 30 to 50%.
This type of erosion occurs subtly as compared to wind and water
erosion and usually results in a redistribution of topsoil within
the field (i.e. the net soil loss is roughly zero, but the net loss
in soil productivity on the knolls can be dramatic). This concept is
reinforced from wheat yields in Idaho (Norris and Comis, 1982).
![](/web/20061121033645im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbe01s08e.jpg)
Figure 7.5 Effect of topsoil on wheat yields in
Idaho |
![](/web/20061121033645im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbe01s08f.jpg)
Figure 7.6 Movement of soil by tillage erosion |
- Tillage erosion occurs only during tillage operations.
- All field operations that disturb the soil cause some
tillage erosion, even operations such as seeding, row crop
cultivation, root crop harvesting, manure injection, etc.
- The heavy duty cultivator moves 10 pounds of soil per foot
width of tillage (15 kilograms of soil per metre) on level land,
but moves 17 to 20 pounds (25 to 30 kilograms) when tilling down
a 15-20% slope and less than 3.5 pounds (less than 5 kilograms)
when tilling up that slope.
- Root crop harvesting can cause more tillage erosion than
plowing.
- The majority of soil moved by tillage is only moved 6 to 12
inches (15 to 30 centimetres), but some soil will be dragged as
far as 6 to 10 feet (2 to 3 metres) and greater distances when
tilling down slope.
- The soil lost from hilltops by tillage erosion is not lost
from the field; it simply accumulates at the bottom of the
hills.
- The soil that accumulates at the bottom of the hills is not
degraded by the erosion process.
- Tillage erosion moves soil down slope to areas where water
erosion is most intense, so tillage erosion is linked to water
contamination.
- Maximum tolerable loss: 5 tons/acre/year (10 tonnes/hectare/year)
= 0.03 inches (0.75 millimetres) thickness of topsoil on a
well-developed soil.
![](/web/20061121033645im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbe01s08g.jpg) |
Figure 7.7
An undisturbed landscape prior to the effects of tillage
erosion |
![](/web/20061121033645im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbe01s08h.jpg) |
Figure 7.8
A cultivated landscape showing the short-term effects of
tillage erosion. Topsoil is being removed from the
knolls and accumulating in the depressions. |
![](/web/20061121033645im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbe01s08i.jpg) |
Figure 7.9
Medium-term effects of tillage erosion, typical of many
prairie landscapes in their current condition. Topsoil
is almost completely removed from knolls and depressions
have thick layers of topsoil due to accumulation.
Yield variability across the landscape is significant. |
![](/web/20061121033645im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbe01s08j.jpg) |
Figure 7.10
Landscape restoration - the practice of moving some of the
accumulated topsoil from depressions back onto the knolls at
a depth of 4 to 6 inches (10 to 15 centimetres) - is
recommended to restore productivity to the knolls and reduce
crop yield variability in the field. |
![](/web/20061121033645im_/https://www.gov.mb.ca/agriculture/soilwater/soil/images/fbe01s08k.jpg) |
Figure 7.11
Long-term effects of tillage erosion. If allowed to
continue, tillage erosion will move subsoil from the knolls
onto the depressions, burying the topsoil and reducing yield
productivity in these areas as well. |
Consult soils report to identify areas prone
to tillage erosion
Tillage erosion has only been recently recognized and, therefore,
it is not clearly reflected in soils reports. However, there is
information in these reports that does help in the identification of
areas prone to tillage erosion as well as wind and water erosion.
- E subclass in the agriculture capability rating for a given
soil series or phase, indicating an erosion limitation.
- T subclass in the agriculture capability rating for a given
soil series or phase, indicating a slope limitation. Land with
steep slopes will have greater rates of both water and tillage
erosion.
- Eroded phases on maps: 1xxx, 2xxx, and 3xxx
often indicate soil loss by tillage erosion, particularly when
those eroded phases appear on hilltops (see examples under wind
and water erosion).
- Slope phases on maps: xbxx to xhxx (see
examples under water erosion). Steeper slopes have greater rates
of tillage erosion. As steepness of slope increases, the
difference in the amount of soil moved down slope by downslope
tillage and up slope by upslope tillage increases.
Site visit to identify areas prone to
tillage erosion
- Land that is hilly is sensitive to tillage erosion. Fields
with many small hills are more prone to tillage erosion than
fields with a few large hills. Hummocky land is more sensitive
to tillage erosion than undulating land and is much more
sensitive than rolling land.
- Land that has shallow topsoil or has areas where topsoil is
shallow, like hilltops, is most sensitive to any form of soil
erosion.
- Even land that is considered to be flat can suffer from
tillage erosion. Tillage erosion is probably the major cause for
the infilling of surface drains in cultivated lands.
- Reduce tillage frequency
All unnecessary tillage operations should be eliminated from a
tillage system. Tillage should be done when soil conditions are
suitable to avoid correctional tillage. If possible, a reduced- or
zero-tillage system should be adopted.
- Reduce tillage intensity
The depth and speed at which a tillage implement is operated
affect its intensity and, therefore, its erosivity. Tillage
implements should be operated at minimum recommended depths and
speeds.
- Reduce tillage speed and depth variability
Operators should try to maintain a constant tillage depth and
tillage speed, even in hilly landscapes. Variability in tillage
depth and speed contributes to tillage erosion.
To maintain constant operating depth and speed in hilly
landscapes requires more power from a tractor than would be
recommended for a specific tillage implement by an equipment
manufacturer or dealer. Implements are rated for required horsepower
assuming that they will be operated on level ground.
Operation in excess of recommended depth and speed results in
greater variation in soil movement, and, consequently, results in
greater tillage erosion.
- Reduce the size of tillage implements
The larger the implement is relative to the size of the hills,
the more rapid the landscape is leveled. Tillage implements which
are very long and/or very wide should be avoided on landscapes which
are highly susceptible to tillage erosion. Some large implements
have flexible frames which allow them to conform to the shape of the
landscape and, therefore, are less erosive.
- Use contour tillage
Where possible, tillage should be conducted along the contour of
the landscape. This will reduce the variation in tillage depth and
speed and, consequently, reduce tillage erosion.
- Use a reversible moldboard plow
Where tillage is conducted on the contour, a
reversible/rollover/two-way moldboard plow can be used to throw the
furrow upslope, leaving a back furrow on the uppermost slope
position. This works against the progressive downslope movement of
soil by other tillage implements (Foster, 1964).
The most effective way to arrest tillage erosion is to eliminate
tillage; however, it is not always desirable to do so. Where tillage
is used, there are practices which can be used to reduce tillage
erosion. Improvements to tillage practices should be made
immediately. Practices which require the purchase of equipment may
or may not provide short-term economic benefits. Individual
Beneficial Management Practices (BMPs) to reduce tillage erosion may
or may not reduce soil loss to tolerable levels.
There are a few additional considerations regarding the reduction
of tillage erosion:
- Where the soil degradation from tillage erosion is a problem
and it is not possible to implement BMPs to reduce tillage
erosion to a tolerable level, it may be advisable to take the
land out of crop production which requires tillage.
- Where it is feasible, areas which are severely degraded by
tillage erosion should be restored by returning the topsoil
which has accumulated downslope and/or by applying amendments
such as livestock manure. This should be followed by the
implementation of BMPs to reduce tillage erosion.
- Field boundaries such as fences and terraces compound soil
losses by tillage erosion and careful consideration should be
given to their placement within the landscape.
Follow-up monitoring
- Measure tillage depth and the amount of surface area
disturbed, taking particular note of depths during upslope
tillage and downslope tillage. Keep records.
- Monitor the speed of all operations that disturb soil,
taking particular note of speeds during upslope tillage and
downslope tillage. Keep records.
- Conduct soil testing for organic matter and nutrient status.
Compare results from hilltops to those from the back of hills
and at the bottom of hills. Track changes over time.
- Use a crop yield monitor and maps to compare results from
hilltops to those from the back of hills and at the bottom of
hills. Track changes over time.
|